Not applicable.
Not applicable.
This invention relates generally to turbines, including microturbines, and more particularly to a method and apparatus to increase the power output of a turbine.
Microturbines are multi-fuel, modular distributed power generation units having multiple applications. They do not require the installation of expensive infrastructure to deliver power to end-users. Thus, in parts of the world lacking the transmission and distribution lines of a basic electric infrastructure, commercialization of microturbines will be an enabling application. In the United States and other countries already having a suitable electric infrastructure, distributed generation units allow consumers of electricity to choose the most cost-effective method of electric service. In many instances microtubrines may be more cost effective than grid power owing to such considerations as energy rates or transmission line losses. In addition to primary power generation, microturbines also offer an efficient way to supply back-up power or uninterruptible power. Other applications exist as well, such as the use of flare gas and landfill gas to generate power. They may also be used to remove harmful chemicals from air that becomes polluted as a result of a manufacturing process.
Current, state of the art, microturbine designs utilize low pressure ratio cycles (3-4 Pr(Pexit/Pinlet)), and rely primarily on recuperation to increase thermodynamic efficiency. In order to increase the power of a microturbine beyond that which can be achieved with increased firing temperature (which, typically, results in a 10-15% increase in power, at the sacrifice of component life), the microturbine must be scaled to a larger aerodynamic flow size. However, scaling is costly, and, while providing additional power, does little if anything to increase system efficiency.
The use of supercharging, in conjunction with intercooling, is another method that may be used to increase power. U.S. Pat. No. 5,553,448 to Farrell, et al., discloses systems that include a low-pressure compressor, a high-pressure compressor downstream of the low-pressure compressor, a combustor, a high-pressure turbine downstream of the combustor, and a low-pressure turbine downstream of the high-pressure turbine. In what Farrell, et al., describe as the xe2x80x9cstandard configuration,xe2x80x9d systems of this type include dual concentric shafts, wherein the high pressure turbine drives the high pressure compressor via a first shaft and the low pressure turbine drives the low pressure compressor by a second shaft (which passes through the first shaft). Farrell, et al., purport to improve on this design by eliminating the dual concentric shafts and driving both the high pressure compressor and the low pressure compressor with the high speed turbine. The high-pressure compressor is driven directly; the low-pressure compressor via a speed-reducing gearbox. Farrell, et al., also incorporate an intercooler between the low pressure compressor and the high speed pressure compressor to reduce the power of the high pressure compressor by, approximately, the amount of power required to run the low pressure compressor.
Multiple compressor stages, such as described above, perform optimally (i.e., with the highest efficiency and mechanical integrity) when run at different shaft speeds. As is well understood in the art, the compression of the first stage squeezes the airflow, reducing the aerodynamically corrected flow (Wc) into the second stage, as given by the following relationship:
Wc=Wa(Tr1)/Pr1,
where Wc is the corrected flow at the exit of the first stage of compression, Wa is the actual physical flow, Tr1 is the temperature ratio across the first stage of compression, and Pr1 is the pressure ratio across the first stage of compression.
For a given flow size, the optimum shaft speed is determined using design conventions that are well understood in the field. As the ratio of corrected flows W1/W2 (in this case Wa/Wc) grows, the optimum shaft speeds for the two compressors becomes progressively dissimilar. Increasing the first stage pressure ratio makes this effect worse. Although increasing the first stage temperature ratio can help to reduce the dissimilarity between optimum shaft speeds by a small amount, a first stage temperature ratio increase also has the consequence of reducing the first stage thermodynamic efficiency, which reduces overall engine efficiency.
Intercooling increases power and offers the capability of increasing engine efficiency by reducing the temperature from the discharge of the first compression stage before the inlet to the second compression stage. This temperature reduction has, at least, three notable consequences. First, it reduces the cold side recuperator inlet temperature, allowing the recuperator to run cooler even at higher cycle pressure ratios. Second, the temperature reduction substantially increases the dissimilarity between optimum shaft speeds, which can necessitate the use of a gearbox or another mechanism to turn the first and second stage compressors at different speeds. Third, it reduces the work of compression. Thus, Farrell, et al., disclose the use of a gearbox between the first and second stages to reduce the shaft speed of the first stage relative to the second. Similarly, U.S. Pat. No. 4,896,499 to Rice discloses the use of a separate low pressure compressor-turbine spool concentrically positioned within a high pressure compressor-turbine spool, the two spools running at different speeds to allow the low pressure compressor to run slower than the high pressure compressor.
The types of arrangements shown in Farrell, et al., and Rice, and other examples of intercooled gas turbine power generation systems, involve the use of additional moving parts (e.g. concentric shafts, variable vane geometry, or gear boxes) and create disadvantageous complexities in coping with the dissimilarity between optimal first and second stage compressor shaft speeds. Such arrangements decrease system efficiency due to additional parasitic losses, such as leakage or friction.
It is therefore an object of the present invention to provide a method of increasing the power output and efficiency of a turbine power generating system without scaling, the use of concentric shafts, variable vane geometry, or gearboxes. More specifically, in the case of microturbines, the present invention is designed to allow a large increase in power (i.e., 3-10 times greater than an existing microturbine), while providing cycle efficiency improvements (i.e., 10%-15%) at a relatively low cost and without requiring scaling of such a microturbine.
It is an additional object of the invention to increase the power output and efficiency of a conventional microturbine power generating system while reducing the system""s cost per unit power.
It is still another object of the invention to provide a method for improving a conventional turbine power generating system by integrating a supercharging compressor stage on the same shaft as the high-pressure compressor stage.
Another object of the invention is to provide a supercharging stage to a conventional turbine power generating system or a microturbine power generating system, by the addition of a low pressure compressor to the same shaft as the high pressure compressor, wherein the low pressure compressor and high pressure compressor are of dissimilar (unlike each other) flow types.
It is also an object of the invention to avoid mechanical limitations of compressor wheels by using a mixed flow compressor or, alternatively, an axial flow compressor, in a supercharging stage that is added to a turbine power generating system or microturbine power generating system that uses a centrifugal high pressure compressor stage.
Still another object of the invention is to improve a conventional microturbine by adding a supercharging stage and intercooling, using a minimal number of additional components.
The above objects of the invention are examples of objects only, and should not be construed to narrow the scope of the invention. Both the above-stated examples and other objects of the invention will be evident from the invention, as described and claimed below.
To increase power in a turbine power generation system, a supercharging stage of compression followed by an intercooler is added to a conventional turbine power generation system. An additional spool consisting of a power turbine directly coupled to a generator is also added to extract power from the system. By utilizing the present invention, power from the conventional system can be increased dramatically (on the order of 3 to 10 times) and efficiency raised 10-15%, while cost per unit power substantially decreases. These advantages are best obtained in a cycle which is also recuperated.
Complexities typically associated with an intercooling stage, such as the use of a gear box or other mechanism to match the optimal shaft speeds of the low and high-pressure compressor shafts, are avoided by the use of dissimilar compressor types. In one embodiment, the low-pressure super-charging compressor is a mixed flow compressor and the high-pressure compressor is a centrifugal flow compressor. In another embodiment, the low-pressure compressor includes one or more axial flow compressors and the high-pressure compressor includes a centrifugal flow compressor.
Use of mixed flow or axial flow compressors in the low pressure compression stage also overcomes the mechanical limitations that would exist with centrifugal compressors, if such centrifugal compressors were used on the same shaft as a supercharging stage of a state-of-the art microturbine today. With intercooling, a centrifugal compressor would not be able to withstand the rotational speed required if it were to be integrated on the same shaft as the high-pressure compressor in a microturbine.